This invention relates generally to a method for preparing a hydrolysate of organoalkoxysilanes, in particular a hydrolysate in which the hydrophobic organic and low polar molecules exhibit excellent solubility, as well as the use of this organoalkoxysilane hydrolysate for obtaining transparent films or substrates, including or not organic molecules and/or inorganic particles, and the applications of these substrates and films in the field of optics, in particular ophthalmic optics.
Below in this request, the organoalkoxysilane hydrolysate will be called organo-silicon sol.
Generally, preparing organo-silicon sols is difficult.
The final properties of the sol and consequently of the derived substrates and/or transparent films depend to a large extent on the sol preparation method, even if the final sol drying/condensation step for obtaining the substrate and/or the film also plays a significant role.
Such sols should exhibit stability properties, i.e. after its preparation, the essential characteristics of the sol (condensation rate, proportion of the various hydrolysed and/or precondensed species, viscosity) do not change or very little with time.
Besides, in the optical field, it has also be sought to obtain organo-silicon sols capable of solubilising low polar, hydrophobic organic additives, in particular for obtaining films of a few microns in thickness.
This latter property of the sol must be preserved during the drying step, i.e. when eliminating solubilisation solvents and during final condensation of the species derived from hydrolysis, so that the additive does not precipitate during this step.
Among organic additives that are particularly interesting in the optical field, photochromic compounds can be mentioned.
The document FR-A-2 704 851 describes a method for preparing an organo-silicon sol in which the following operations are conducted: complete hydrolysis of a solution containing one or several organo-alkoxysilanes in an organic solvent or mixtures of organic solvents using an acid aqueous solution with a pH equal to or smaller than 3, elimination of the organic solvent(s) and of the residual alcohols and concentration of the solution by distillation for obtaining a sol.
However, the method of the patent FR-A-2 704 851 leads to sols certain properties of which strongly vary with time, in particular the condensation rate and the composition of the species present in the sol.
Moreover, it is difficult to solubilise in the sols of patent FR-A-2 704 851 low polar, hydrophobic organic additives and in particular photochromic compounds.
The article entitled “Organosiloxane Resin with High Silanol Content” Furuya et al.—Silicones in Coatings II—A Technology Forum Exploring the Versatility of Silicone—24–26 Mar. 1998—Florida—USA—Conference Papers”, describes the synthesis of an organosiloxane resin by hydrolysing trialkoxysilanes with acidified water in the absence of an organic solvent. The alcohol produced during hydrolysis is eliminated by heating or under reduced pressure in order to precipitate a viscous product that is a siloxane resin with high silanol content.
Although the method of the article leads to more stable sols, it would be nevertheless desirable to obtain sols with increased stability as well as better solubility of additives such as photochromic compounds.